The invention relates to a triggering mechanism for a triggering valve of a hydraulic emergency unlock circuit for unlocking hydraulically operated gripping members of a riser connector which connects a lower end of a riser to a wellhead on a sea floor.
Drilling of offshore hydrocarbon wells is performed by a drill string arranged in a riser extending from a wellhead on the sea floor to a drilling vessel. The drilling vessel may be anchored to the sea floor or kept in position by thrusters of a dynamic positioning system. The lower end of the riser is connected to the wellhead by a riser connector, which includes some type of hydraulically operated gripping members, such as fingers which in a locked position clamp a flange of the lower end of the riser.
Connectors which may be used for connecting a riser to a wellhead is disclosed in U.S. Pat. No. 4,721,132, U.S. Pat. No. 5,382,056 and U.S. Pat. No. 6,234,252.
In order to allow a movement of the drilling vessel, which may be caused by wind, waves and current, the riser is normally connected to the riser connector via a flex joint which allows some angular displacement of the riser. If the angular displacement of the riser exceeds a maximum acceptable angle, dictated by maximum allowable stresses in the wellhead, the riser or the drilling vessel, the riser will be disconnected from the wellhead, which is carried out by a hydraulic circuit which unlock the hydraulically operated gripping members of the riser connector.
The hydraulic circuit which unlocks the hydraulically operated gripping members may fail, and it is therefore preferred to have an emergency unlock circuit for unlocking the gripping members. Further, to ensure a high reliability, the activating of the emergency unlock circuit should be reliable and independent of any external connections or signals.
The object of the invention is thus to provide a triggering mechanism for a triggering valve of a hydraulic emergency unlock circuit for unlocking hydraulically operated gripping members of a riser connector which connects a lower end of a riser to a wellhead on a sea floor, which triggering mechanism shall be reliable, and the triggering shall be independent of any external connections or signals.
The object is achieved by a triggering mechanism according to claim 1.
The inventive triggering mechanism is used together with a riser connector as discussed above. Angular displacement of the riser is allowed by a flex joint comprising a stationary part clamped by the gripping members and an angularly displaceable part secured to the lower end of the riser. The stationary part of the flex joint and the displaceable part of the flex joint are coaxial when the riser is in a non-displaced position.
According to the invention, the triggering mechanism comprises a cam ring which is secured to the displaceable part of the flex joint, and which is coaxial with the displaceable part of the flex joint. Further the triggering mechanism comprises hydraulic cylinders which are secured to the stationary part of the flex joint, which are parallel with an axis of the stationary part of the flex joint, and which are arranged in a circle which is coaxial with the stationary part of the flex joint. The hydraulic cylinders have through-going piston rods, and an actuator ring is mechanically connected to ends of the piston rods which point towards the cam ring. The actuator ring is thus coaxial with the stationary part of the flex joint, and thus the cam ring and the actuator ring are parallel when the stationary part of the flex joint and the displaceable part of the flex joint are coaxial. The actuator ring have a clearance to the cam ring. Pistons divide the hydraulic cylinders in upper and lower chambers, the upper chambers being on the actuator ring side of the pistons, the through-going piston rods ensure equal cross-sectional area in the upper and lower chambers. Further cross-connecting conduits connect upper chambers with lower chambers in oppositely located hydraulic cylinders. A trigger for the triggering valve is located on the stationary part of the flex joint, between the stationary part of the flex joint and the actuator ring.
An angular displacement of the riser will cause an angular displacement of the cam ring. When the angular displacement of the cam ring exceeds a predetermined angle, which may happen in an emergency situation, the cam ring contacts the actuator ring and forces the actuator ring towards the stationary part of the flex joint, which causes movement of the piston rods and the pistons. The movement of the pistons causes hydraulic flow in the cross-connecting conduits, which ensure equal movement of pistons and piston rods in oppositely located hydraulic cylinders. The actuator ring thereby moves translatory, and activates the trigger for the triggering valve, irrespectively of the direction of the angular displacement of the riser.
It is thereby provided a triggering mechanism made by simple mechanical and hydraulic components, which make the mechanism reliable. Further the triggering is caused by a direct and simple mechanical transfer of the angular displacement of the riser, independent of any external connections or signals.